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Equivalent Weight: A Key Concept in Environmental & Water Treatment

In the world of environmental and water treatment, understanding the concept of equivalent weight is crucial for effective chemical application and process design. This seemingly simple concept holds significant implications for calculating chemical dosages, predicting reaction outcomes, and optimizing treatment efficiency.

What is Equivalent Weight?

Equivalent weight (EW) is a measure of the reactive capacity of a substance in a specific chemical reaction. It represents the weight of a compound that contains one gram-equivalent of the reactive species, such as hydrogen ions (H+) or hydroxyl ions (OH-).

In simpler terms, it tells us how much of a substance we need to react completely with a fixed amount of another substance.

Calculating Equivalent Weight:

The equivalent weight of a compound is calculated by dividing the molecular weight of the compound by the number of reactive species present in the compound.

For example:

  • Calcium hydroxide (Ca(OH)2): Molecular weight = 74 g/mol. It has two hydroxyl ions (OH-), so its equivalent weight is 74/2 = 37 g/mol.
  • Sulfuric acid (H2SO4): Molecular weight = 98 g/mol. It has two hydrogen ions (H+), so its equivalent weight is 98/2 = 49 g/mol.

Applications in Environmental & Water Treatment:

Equivalent weight plays a vital role in various aspects of environmental and water treatment:

  • Chemical Dosage Calculation: It helps determine the exact amount of chemical reagent needed to achieve a desired reaction in water treatment processes like coagulation, flocculation, and disinfection.
  • Understanding Stoichiometry: It enables accurate predictions of reaction products and byproducts, ensuring optimal treatment effectiveness and minimizing unwanted side reactions.
  • Optimizing Treatment Efficiency: By considering equivalent weight, we can optimize the use of chemicals, minimizing costs and environmental impact.
  • Process Design: It aids in the design and operation of water treatment systems by allowing accurate calculation of chemical feed rates and reactor sizes.

Examples:

  • Coagulation: Alum (Al2(SO4)3) is a commonly used coagulant. Its equivalent weight helps calculate the precise dosage needed to neutralize the negative charges on suspended particles in water, promoting their coagulation and removal.
  • Disinfection: Chlorine (Cl2) is used to disinfect water. Its equivalent weight helps determine the amount of chlorine needed to achieve a specific disinfection level, ensuring the elimination of harmful pathogens.
  • Acidification/Alkalinization: Lime (Ca(OH)2) is often used to adjust pH in water treatment. Its equivalent weight helps calculate the amount needed to neutralize the acidity or alkalinity of water, achieving the desired pH level.

Conclusion:

Equivalent weight is a fundamental concept in environmental and water treatment. It provides a powerful tool for understanding chemical reactions, optimizing chemical usage, and designing effective treatment processes. By embracing this concept, we can ensure the delivery of safe, clean water and minimize the environmental footprint of our treatment operations.

Test Your Knowledge

Equivalent Weight Quiz:

Instructions: Choose the best answer for each question.

1. What does "equivalent weight" represent?

a) The mass of a compound containing one mole of the substance. b) The weight of a compound that contains one gram-equivalent of the reactive species. c) The molar mass of a compound divided by its density. d) The weight of a compound that reacts with one gram of water.

Answer

b) The weight of a compound that contains one gram-equivalent of the reactive species.

2. How do you calculate the equivalent weight of a compound?

a) Divide the molecular weight by the number of reactive species. b) Multiply the molecular weight by the number of reactive species. c) Subtract the number of reactive species from the molecular weight. d) Add the number of reactive species to the molecular weight.

Answer

a) Divide the molecular weight by the number of reactive species.

3. What is the equivalent weight of calcium hydroxide (Ca(OH)2)?

a) 37 g/mol b) 74 g/mol c) 148 g/mol d) 296 g/mol

Answer

a) 37 g/mol

4. In water treatment, equivalent weight helps determine:

a) The amount of chemical needed for a desired reaction. b) The efficiency of water filtration systems. c) The level of dissolved oxygen in water. d) The color of treated water.

Answer

a) The amount of chemical needed for a desired reaction.

5. Which of the following is NOT an application of equivalent weight in environmental and water treatment?

a) Calculating the amount of chlorine needed for disinfection. b) Designing the size of water treatment tanks. c) Determining the optimal pH for water. d) Measuring the turbidity of water.

Answer

d) Measuring the turbidity of water.

Equivalent Weight Exercise:

Scenario: A water treatment plant uses sodium hydroxide (NaOH) to adjust the pH of water. The desired pH is 8.5, and the plant needs to treat 10,000 gallons of water.

Task: Calculate the amount of sodium hydroxide (NaOH) needed to achieve the desired pH, given the following information:

  • Equivalent weight of NaOH = 40 g/mol
  • The initial pH of the water is 7.0
  • The water has a hardness of 100 mg/L as CaCO3
  • Assume that the hardness is primarily due to calcium ions (Ca2+)

Hints:

  • You will need to use the concept of equivalent weight to determine the amount of NaOH needed to neutralize the calcium ions.
  • Consider the chemical reaction between NaOH and Ca2+.
  • Use the equivalent weight of CaCO3 (50 g/mol) to convert hardness from mg/L as CaCO3 to mg/L as Ca2+

Exercise Correction:

Exercice Correction

  1. Convert hardness from mg/L as CaCO3 to mg/L as Ca2+:

    • The molar ratio of CaCO3 to Ca2+ is 1:1.
    • The equivalent weight of CaCO3 is 50 g/mol, and the equivalent weight of Ca2+ is 20 g/mol (atomic weight of Ca/2).
    • Therefore, hardness in mg/L as Ca2+ = (hardness in mg/L as CaCO3) * (equivalent weight of Ca2+ / equivalent weight of CaCO3) = 100 mg/L * (20 g/mol / 50 g/mol) = 40 mg/L as Ca2+

  2. Calculate the moles of Ca2+ in the water:

    • 10,000 gallons of water = 10,000 gallons * 3.785 liters/gallon = 37,850 liters
    • Moles of Ca2+ = (40 mg/L * 37,850 L) / (40 g/mol * 1000 mg/g) = 37.85 moles

  3. Calculate the moles of NaOH needed to neutralize the Ca2+:

    • The chemical reaction is: 2NaOH + Ca2+ → Ca(OH)2 + 2Na+
    • The mole ratio of NaOH to Ca2+ is 2:1.
    • Moles of NaOH = 37.85 moles * 2 = 75.7 moles

  4. Calculate the mass of NaOH needed:

    • Mass of NaOH = 75.7 moles * 40 g/mol = 3028 g

  5. Convert the mass of NaOH to kilograms:

    • Mass of NaOH = 3028 g * (1 kg / 1000 g) = 3.028 kg

Therefore, approximately 3.028 kg of sodium hydroxide (NaOH) is needed to adjust the pH of 10,000 gallons of water to 8.5.

Books

  • Chemistry: The Central Science by Theodore L. Brown, H. Eugine LeMay Jr., and Bruce E. Bursten: This widely-used textbook provides a comprehensive introduction to chemistry, including the concept of equivalent weight.
  • Water Treatment Engineering by AWWA (American Water Works Association): This comprehensive book covers various aspects of water treatment, including chemical dosage and reaction stoichiometry, where equivalent weight is crucial.
  • Environmental Engineering: A Global Text by Davis and Cornwell: This textbook explores environmental engineering principles, including chemical treatment processes, highlighting the role of equivalent weight in practical applications.

Articles

  • "Equivalent weight: A forgotten concept in chemical engineering" by R.A. Jones and D.R. Lloyd: This article discusses the importance of equivalent weight in chemical engineering calculations and its relevance to environmental and water treatment.
  • "The Role of Equivalent Weight in Water Treatment Processes" by S.R. Sharma: This article emphasizes the applications of equivalent weight in various water treatment processes, including coagulation, disinfection, and pH adjustment.
  • "Calculating Chemical Dosages for Water Treatment using Equivalent Weight" by J.S. Lee: This article provides a practical guide on calculating chemical dosages in water treatment using the concept of equivalent weight.

Online Resources

  • Wikipedia: Equivalent weight - Provides a detailed explanation of the concept and its relevance in various fields.
  • Chemistry LibreTexts: Equivalent Weight - Offers a comprehensive explanation of equivalent weight with examples and applications.
  • Purdue University: Equivalent Weight - Presents a clear and concise explanation of equivalent weight with illustrations and examples.

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